1. Field of the Invention
The invention relates generally to sensor systems and, more particularly, to a system capable of fast-response, high resolution, high accuracy three-dimensional position measurement using magnetic sensors.
2. Description of the Related Art
Various systems have been proposed for detecting the position and/or orientation of an object using magnetic or electromagnetic fields. These systems typically employ field transmitters, such as electromagnet coils, disposed at known locations or in a fixed reference frame, and a sensor, such as a coil or other transducer mounted to the object to be located. Each transmitter projects a field varying in space in a fixed frame of reference. The pattern of variation in space for each transmitter is different than the pattern of variation for each other transmitter.
For example, the transmitters may be identical to one another but disposed at different locations or in different orientations. The field patterns of the transmitters are thus displaced or rotated relative to one another, and relative to the fixed frame of reference. The sensor on the object detects the parameters of the field prevailing at the location of the object as, for example, the magnitude and/or direction of the field at the object or the magnitude of individual components of the field at the object in one or more pre-selected directions. The transmitter may be actuated in a predetermined sequence so that at any time only one transmitter is active and therefore the field prevailing at the object is only the field contributed by one transmitter, plus a background field due to the Earth""s magnetic field and other environmental sources.
Alternatively, the transmitters can be driven at different frequencies so that components of the signal from the sensor varying at different frequencies represent contributions to the field at the object from different transmitters simultaneously. Based upon the detected parameters of the fields from the individual transmitters, and the known pattern of variation of the field from each transmitter, a computer system calculates the position and orientation of the sensor, and hence the position of the object bearing the sensor, in the fixed frame of reference of the transmitters. In a variant of this system, the object to be located carries the transmitters, whereas a plurality of sensors are disposed at various locations or orientations in the fixed frame of reference. The location and/or orientation of the object is deduced from signals representing the parameter of the field prevailing at the various sensors.
Systems of this general nature are disclosed in U.S. Pat. Nos. 4,849,692; 4,642,786; 4,710,708; 4,613,866 and 4,945,305. Systems according to this general design can be used to provide a three-dimensional spatial input capability for a computer.
U.S. Pat. No. 4,054,881 to Raab discloses three mutually orthogonally radiating antennas each of which transmits electromagnetic radiation to three mutually orthogonal receiving antennas. The receiving antennas measure the radiated signals and produce nine parameters which enable the calculation of the position and orientation of the receiving antennas. No mention is made of determining the velocity or acceleration of the sensor.
U.S. Pat. No. 4,622,644 to Hansen discloses a system which enables the measurement and position of a permanent magnet within a three-dimensional region in five degrees of freedom. In order to sense the position and orientation of the permanent magnet, the system uses three antennas each composed of three mutually orthogonal Hall effect devices. The output voltages from the nine Hall effect devices are inputted into a microprocessor device which first calculates an estimate of the position using a nonlinearized algorithm. Subsequently, the microprocessor uses a linearized algorithm to calculate the precise position and orientation of the permanent magnet. Hansen""s device uses Hall sensors, rather than transmitter coils, as the primary sensors.
U.S. Pat. No. 5,729,129 to Acker discloses a system utilizing three electromagnetic transmitters arranged about an electromagnetic detector positioned in the chest of a patient (FIG. 1). The transmitting coils are all arranged in a common plane.
U.S. Pat. No. 5,712,478 to Ollson discloses a system of general interest which utilizes three transmitting and three receiving means. Ollson""s system is used to measure the position of a ball joint.
U.S. Pat. No. 5,257,676 to Merkley et al discloses an apparatus for stabilizing levitated objects. Merkley""s system is of interest with respect to its teachings relating to position and velocity determination of levitated objects.
These systems are in need of improvement due to several deficiencies in the pertinent art. First, some inventions use a limited number of transmitters and thus can operate over only a limited range. Second, some inventions require a special orientation of the transmitters and/or receivers and are thus limited in applications where such special orientation conditions are not feasible. Third, because the magnetic fields from typical transmitters (i.e., dipole field) vary approximately as the inverse cube of distance from the transmitter, the signal detected by the receiver is very small at long range, yielding a limited effective range of operation and/or the resolution/accuracy of the sensor is also limited.
To overcome the range limitations, received signals from a sensor are time-averaged to reduce environmental and electronic noise effects. The time-averaging increases the signal to noise ratio (SNR), yielding improved range, resolution, or accuracy (or all of the above). The trade-off is that such time-averaging slows the sensor""s response time, so that the sensor cannot respond to sudden positional changes (i.e., velocity or acceleration).
Further, in some applications (e.g., car crash test dummy applications), the desired parameter is not just position, but speed and/or acceleration of the test object. To calculate velocity and acceleration, one must differentiate the position data resulting in noisy data or reduced accuracy/resolution.
Accordingly, it is an object of the present invention to provide a system and process that overcome the deficiencies of the art. There is provided according to the invention a magnetic sensor system for determining the three-dimensional position, velocity and acceleration of an object utilizing magnetic field currents, said sensor system being capable of operating within close proximity to metal surfaces and metal objects, comprising an object, the position, velocity and acceleration of which are to be determined; a three-dimensional fixed reference frame of known dimensions, wherein said object is located within said fixed reference frame; a power source capable of generating a magnetic field within said fixed reference frame; a plurality of magnetic field transmitters, said transmitters operatively interconnected to said power source and capable of being geometrically arbitrarily oriented relative to said fixed reference frame; at least one magnetic field receiver, said receiver capable of receiving electronic signals from said transmitters and further capable of being geometrically arbitrarily oriented relative to said fixed reference frame; a programmed computer, said computer capable of receiving said signals from said receiver and further capable of calculating the position, velocity and acceleration of said object based upon said signals.
There is also provided according to the invention a method for determining the position, velocity and acceleration of an object, comprising providing a three dimensional fixed reference frame of known dimensions; providing an object, the position, velocity and acceleration of which are to be measured; generating electrical current from an oscillator; delivering said current from said oscillator to a power amplifier; directing said amplified current from said amplifier to a plurality of transmitters; generating a magnetic field from said transmitters in said reference frame; receiving said magnetic field signal from said transmitters into at least one receiver; demodulating and amplifying said received magnetic field signal into magnetic field components from said receiver signal, wherein said output from said amplifier is proportional to said magnetic field components; applying a mathematical filter to said demodulated and amplified signal; and applying a mathematical algorithm to calculate the position, velocity and acceleration of said object.
Further objects, features, and advantages of the invention will become apparent from the detailed description that follows.